Current density effects on the electrical reliability of ultra fine-pitch micro-bump for TSV integration

Flip chip solder bump has been widely used in the electronics industry in recent years as high performance and miniaturized electronics have become more common. Microbump is one of candidates to solve reliability issues because it provides the fine pitch and uniform current distribution. However, electromigration of the microbump has recently been an important reliability issue of flip chip packages. There are several important issues such as current crowding, polarity effect, thermomigration. And excessive intermetallic compound (IMC) growth in microbump can degrade the mechanical reliability of solder joints. Therefore, it is essential to understand the fundamental growth mechanisms of IMC. Also, temperature and current are major parameters that impact electromigration reliability. Due to the large current used in the accelerated electromigration test, the joule heating associated with the stress current can be significant. In this study, annealing and current stressing conditions were performed at 120°C, 150°C, and 165°C with 1.5×10⁵ A/cm² in order to investigate the IMC growth kinetics in fine-pitch Cu/Sn-Ag microbump by using in-situ scanning electron microscope (SEM). And current density effects on the IMC growth kinetics in microbump for TSV integration were quantitatively evaluated. In the Sn-limited system, two different IMC growth stages were existed. Under electric current stressing, the IMCs growth is accelerated by the influence of the electron wind force, and the IMC phase transition time became shorter as IMC growth rates increased. After all Sn was transformed to IMCs, current stressing effect on IMC growth rate were negligible due to less current crowding and Joule heating effects as well as slow diffusion rate inside Cu-Sn IMC of fully IMC-transformed Cu/Sn microbump, which means much stronger electromigration resistance of Cu/Sn microbump compared to conventional solder bump.